1. Field of the Invention
The present invention relates generally to laser communication receivers, and more particularly to a wideband heterodyne receiver for a laser communication system.
2. Background of the Invention
Considerable interest exists in laser communication links as they provide an efficient, wide bandwidth, relatively noise immune technique of transmitting information, particularly in open environments such as outer space. Specific situations in which laser communication links would be most beneficial are in satellite to earth station communications and satellite to satellite communications. In particular, a number of advantages, clearly apparent to those skilled in the art, would be derived from a system which would permit interference free communication between a synchronous satellite and a low altitude satellite and also between the low altitude satellite and a ground station. The most serious problem in the development of such a communication system is in the high relative velocities between the various stations involved resulting in a large, time varying doppler frequency shift in the communication system.
In a laser communication system, such a time varying doppler frequency shift must be frequency tracked by either a spectrally a wide laser local oscillator which can be rapidly tuned at infrared wavelengths, or by a heterodyne receiver which employs a fixed frequency laser local oscillator and a wideband intermediate frequency network which can be rapidly scanned using radio frequency techniques.
The latter technique has been used in constructing a heterodyne laser communications receiver in accordance with the teachings of the present invention, as will be set forth in more detail subsequently.
Other optical heterodyne communication receivers are known to those skilled in the art as evidenced by U.S. Pat. No. 3,530,298 to Hubbard, et al, issued Sept. 22, 1970. However, while the system disclosed in the Hubbard et al patent includes a wideband optical heterodyne receiver with appropriate pulse processing circuitry, it does not incorporate any doppler frequency shift circuitry, appropriate photomixers and other signal processing components which would render it suitable for use in satellite-to-satellite or satellite-to-ground communications of the type briefly described above. Accordingly there is a need for an improved laser communication receiver which is capable of compensating for large doppler frequency shifts in an environment where a large relative velocity exists between transmitting and receiving stations.